This invention relates to a process for the selective hydrogenation of dienes from a mixed hydrocarbon stream.
Recent legislation, including the Clean Air Act, has made gasoline reformulation a priority. The driving force continues in research to try to find ways to produce cleaner burning fuels to minimize harm to the environment. Motor fuel components that comprise high concentrations of undesirable elements are under scrutiny as members of the public and private sectors strive to reduce or eliminate such elements.
One of those fuel components under scrutiny is the naphtha stream coming from the fluid catalytic cracker unit (xe2x80x9cFCCUxe2x80x9d) in the refinery. FCCU naphtha is the source of many elements alleged to be harmful to the environment and generally contains significant amounts (greater than 1%) of conjugated dienes and alkynes, as well as sulfur. It is desirable to reduce or eliminate these components in FCCU naphtha before it is introduced to the next stage of refinery processing, for example, to an etherification or alkylation unit.
FCCU naphtha may be used as the feed stream to an etherification unit in order to make tertiary amyl methyl ether (TAME). TAME, used in reformulated gasoline, boosts the octane rating of gasoline. When FCCU naphtha is used as a feed stream for the production of TAME, the dienes and alkynes present in the feed share a tendency to polymerize on the reaction catalyst, causing catalyst deactivation. This results in having to shut down the unit and regenerate or replace the catalyst, which is costly both in terms of production time and catalyst. Therefore, it is important to minimize the amount of dienes and alkynes in the feedstream to the etherification unit.
Another important use for the FCCU naphtha stream is to feed an alkylation unit in a refinery. Alkylation produces alkylate, one of the best gasoline blending components because of its high octane and low vapor pressure. As with etherification, feed impurities adversely affect the alkylation catalyst, causing deactivation. In an alkylation process, the reaction catalyst is an acid. The amount of impurities, such as dienes, in the feed dictate the makeup requirements for the acid, such that a certain amount, n, of impurities in the feed will require an amount of additional makeup acid, i.e. some multiple of n, for the reaction to effectively proceed. FCCU naphtha often contains approximately about 1% or more of dienes. Even this small amount of dienes in the feed stream to the alkylation process has been widely recognized as being undesirable. For a general description of an alkylation process, reference is made to Meyers, Handbook of Petroleum Refining Processes, 2d Edition, 1997, pages1.3-1.7.
Butadiene is a common impurity in alkylation plant feeds. Butadiene tends to polymerize and form acid soluble oils, which increase acid makeup requirements. As butadiene levels resulting from catalytic cracking operations tend to be high, it is important to selectively hydrogenate the dienes and the diolefins without hydrogenating the valuable monoolefins.
As described above, the diolefinic elements in the feed streams to the etherification and alkylation units cost time and money. It is therefore desirable to convert the diolefinic components in the cracked gasoline to monoolefinic components prior to introducing the feed to an etherification or alkylation unit.
It is well known that diene-containing cracked gasoline and other light cracked hydrocarbon oils can be selectively hydrogenated by passing these materials over a suitable hydrogenation catalyst in the presence of hydrogen. It is also well known that suitable hydrogenation catalysts contain nickel, molybdenum, palladium, and other metals, including precious metals, supported on a carrier. The prior art, however, indicates that when using a one of the aforementioned hydrogenation catalysts that the catalysts are preferably sulfided or otherwise pretreated with sulfur to moderate activity and increase selectivity.
For example, U.S. Pat. No. 3.234,298 to Langhout et al. teaches the use of sulfided nickel on alumina catalyst for a selective hydrogenation of diene-containing cracked hydrocarbons. U.S. Pat. No. 3,472,763 to Cosyns et al. teaches that a nickel oxide catalyst on alumina may be used for selective hydrogenation, and that it is preferable to pretreat the catalyst with a sulfur compound and specifies pore size distributions. U.S. Pat. No. 3,919,341 to Germanas et al. is pertinent for its teaching of a sulfided nickel on alumina composite. U.S. Pat. No. 3,301,913 to Holmes and Pitkethly teach the use of a nickel catalyst wherein a portion of the nickel is combined with sulfur. U.S. Pat. No. 4,227,993 to Englehard et al. discloses the use of catalyst comprising platinum, tin and other metals wherein it is advantageous to presulfide the catalyst to minimize the cracking reactions which tend to occur at the start of the treatment.
Other references in the prior art teach the introduction of a component in addition to the feed stream to facilitate selective hydrogenation and to prevent catalyst deactivation. For example, U.S. Pat. No. 3,662,015 to Komatsu et al. teaches a nickel on alumina selective hydrogenation catalyst in which the process includes passing an amount of carbon monoxide along with the feed stream and hydrogen to the hydrogenation reaction zone. U.S. Pat. No. 3,900,526 to Johnson et al. teaches the use of metallic arsenide and antimonide hydrogenation catalysts, with the option of introducing carbon monoxide as a modifier. U.S. Pat. No. 3,238,269 to Holmes teaches the addition of a sulfur-containing compound to the feed stream mixture.
All of the aforementioned U.S. patents and literature references are incorporated herein by reference.
As can be seen from the above, the art is replete with hydrogenation processes. As can also be seen from above, the addition of sulfur to either the catalyst or to the feed stream to promote the hydrogenation process is common. However, the addition of sulfur to the catalyst or to the feed stream to maintain catalyst activity can create an unnecessary sulfur removal problem when the selectively hydrogenated products are to be further processed in other catalytic reactors in which minute amounts of sulfur are detrimental. Thus, it would be desirable to provide a selective hydrogenation process that does not require additional sulfur, either as a component in addition to the feed stream or to the catalyst.
In addition to limiting the amount of sulfur involved in the hydrogenation process, the necessary elements for commercial success of any such process include keeping the process as simple as possible, reducing the costs of the necessary materials, i.e. the catalyst, and making the process as efficient as possible in hydrogenating mixed hydrocarbon feed streams.
Accordingly, it would be desirable to provide a selective hydrogenation process in which, to save time and money, the catalyst does not require an activation or pre-treatment step.
It would also be desirable to provide a selective hydrogenation process in which a readily available and economically efficient catalyst were used in order to avoid the use of precious metal catalysts, which are expensive and susceptible to deactivation due to sulfur contamination.
Further, it would be desirable to provide a selective hydrogenation process in which the catalyst is resistant to typical catalyst poisons such as sulfur and which can be used for sustained periods without significant regeneration.
It would also be desirable to provide a selective hydrogenation process wherein conjugated dienes in FCCU naphtha are at least 80 to 90% removed.
Accordingly, the inventors have discovered the use of a commercially available catalyst that is suitable for selective hydrogenation of dienes such that conjugated dienes are removed to below a readily detectable limit, wherein the catalyst is not expensive, and does not require any activation or pretreatment step, wherein sulfur normally present in the feed stream presents no adverse effect, and no component in addition to the feed stream (and a hydrogen environment in a reaction zone) are necessary for hydrogenation. With these and other important discoveries, the present invention provides a significant improvement over the prior art.
In accordance with the present invention, there is provided a novel process for the selective hydrogenation of conjugated dienes in a mixed hydrocarbon stream comprising essentially hydrocarbons as would be found in a fluid catalytic cracking unit (FCCU) naphtha stream, wherein activation or pre-treatment of the catalyst is not required and the removal of sulfur, a potential catalyst poison, from the feed stream is not required.
According to one aspect of the invention, a reaction zone is established having a hydrogenation catalyst. The catalyst may comprise nickel on an aluminum oxide support or nickel oxide and molybdenum oxide on an alumina support. A mixed hydrocarbon feed stream is introduced to the reaction zone and passed over the catalyst. Hydrogen is introduced as a co-feed to the reaction zone. The reaction zone is operated at temperature and pressure conditions to effect the selective hydrogenation of conjugated dienes. Finally, an effluent stream, in which at least 80 to 90% of the conjugated dienes have been removed, is recovered from the reaction zone.